users-guide: Fix various issues in debugging flags section
users-guide: Fix various issues in debugging flags section

Reviewers: ggreif

Reviewed By: ggreif

Subscribers: rwbarton, thomie, carter, ggreif

Differential Revision: https://phabricator.haskell.org/D4750

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc